Rajinesque  on  Atmospheric  Dust*  397 

is  sufficient  to  entitle  them  to  the  lasting  gratitude  of  our  pro- 
fession.  It  was  one  of  the  merits  of  that  illustrious  physician 
of  our  own  time  and  country,  Dr.  Rush,  that  he  seized  with 
avidity  every  fact,  from  whatever  quarter  it  might  be  drawn^ 
to  elucidate  his  favourite  science.  If  ever  medicine  shall  attain 
to  the  elevation  of  a  truly  philosophical  science^  it  must  be  ac¬ 
complished,  in  part  at  least,  by  imitating  his  example,  and  by 
developing  the  infinite  and  diversified  associations  which  exist 
between  it  and  the  other  sciences. 


Art.  XIV.  Thoughts  on  Atmospheric  Dust*  By  C.  S. 

Rafinesque,  Esq* 

“  When  we  find  the  ruins  of  ancient  cities  buried 
under  ground ;  when  the  plough  uncovers  the  front  of  pa¬ 
laces  and  the  summit  of  old  temples,  we  are  astonished  :  but 
we  seldom  reflect  why  they  are  hidden  in  the  earth.  A  sort 
of  imperceptible  dust  falls  at  all  times  from  the  atmosphere, 
and  it  has  covered  them  during  ages.” 

2.  These  are  the  words  of  the  worthy  and  eloquent  philo- 
!  sopher  Virey,  in  his  article  Nature,  Vol.  XV.  p.  373,  of  the 
I  French  Dictionary  of  Natural  History.  Even  before  reading 
[  them  I  had  observed  the  same  phenomenon,  and  I  have  since 

studied  their  eflects  in  various  places.  I  could  quote  one 
thousand  instances  of  the  extensive  and  multifarious  operations 
of  this  meteoric  dust :  but  I  mean  to  give  the  results  merely 
of  those  that  fall  daily  under  notice,  and  are  yet  totally  ne¬ 
glected  ;  wishing  to  draw  on  them  the  attention  of  chemists, 
philosophers,  and  geologists. 

3.  Whenever  the  sun  shines  in  a  dark  room,  its  beams  dis¬ 
play  a  crowd  of  lucid  dusty  molecules  of  various  shapes, 
which  were  before  invisible  as  the  air  in  which  they  swim, 
but  did  exist  nevertheless.  These  form  the  atmospheric 
dust ;  existing  every  where  in  the  lower  strata  of  our  atmos¬ 
phere.  I  have  observed  it  on  the  top  of  the  highest  moun- 

VoL.  I. ...No.  4.  31 

. 


398 


Rajinesque  on  Atmospheric  Dust* 


tains,  on  Mount  Etna,  in  Sicily,  on  the  Alps,  on  the  Alleghany 
and  Catskill  mountains  in  America,  &c.  and  on  the  ocean. 

4.  It  deserves  to  be  considered  under  many  views  :  which 
are  its  invisibility,  its  shape  and  size,  its  formation  and  origin, 
its  motion,  its  deposition  and  accumulation,  its  composition, 
its  uses,  and  its  properties. 

5.  This  dust  is  invisible,  owing  to  the  tenuity  of  its  parti¬ 
cles,  but  they  become  visible  in  the  following  instances ; 
when  the  sun  shines  on  them,  since  they  reflect  the  light, 
when  their  size  is  increased,  and  when  they  are  accumulated 
any  where. 

6.  The  size  of  the  particles  is  very  unequal,  and  their 
shape  dissimilar  ;  the  greatest  portion  are  exceedingly  small, 
similar  to  a  whitish  or  grayish  spark,  without  any  determina¬ 
ble  or  perceptible  shape  ;  the  larger  particles  are  commonly 
lamellar  or  flattened,  but  with  an  irregular  margin,  and  the 
largest  appear  to  be  lengthened  or  filiform  ;  the  gray  colour 
prevails.  Other  shapes  are  now  and  then  perceptible  wfith 
the  microscope. 

7.  Among  the  properties  of  atmospheric  dust  are  those  of 
being  soft,  as  light  as  atmospheric  air,  of  reflecting  the  rays 
received  directly  from  the  sun,  of  possessing  a  kind  of  pecu¬ 
liar  electricity,  which  gives  it  a  tendency  to  accumulate  on 
some  bodies  more  readily  than  on  some  others,  and  of  forming 
an  earthy  sediment,  which  does  not  become  effervescent  with 
acids. 

8.  This  dust  is  either  constantly  or  periodically  formed,  but 
chemically  in  the  atmosphere  like  snow,  hail,  meteoric  stones, 
honey-dew,  earthy  rains,  &c.  by  the  combination  of  gaseous 
and  elementary  particles  dissolved  in  the  air.  Its  analysis 
has  never  been  attempted  by  chemists ;  but  the  earthy  sedi¬ 
ment  which  is  the  result  of  its  accumulated  deposition,  proves 
that  it  is  a  compound  of  earthy  particles  in  a  peculiar  state  of 
aggregation,  and  in  which  alumine  appears  to  preponderate, 
rather  than  calcareous  or  silicious  earths  or  oxides. 

9.  Its  motion  in  calm  weather,  or  in  a  quiet  room,  is  very 
slow  ;  the  particles  appear  to  float  in  the  air  in  all  directions, 
some  rising,  some  falling,  and  many  swimming  horizontally,  or 


Rqfinesque  on  Atmospheric  Dust.  399 

forming  a  variety  of  curved  lines  ;  what  is  most  singular,  is 
that  no  tw(i  particles  appear  to  have  exactly  the  same  direc¬ 
tion  ;  yet  after  awhile  the  greatest  proportion  fall  down  ob¬ 
liquely,  somewhat  in  the  same  manner  as  a  light  snow  in  a 
^  calm  day.  When  a  current  of  air  is  created  naturally  or 

artificially  in  the  open  air  or  in  a  room,  you  perceive  at 
once  an  increased  velocity  in  their  motion  ;  they  move  with 
rapidity  in  all  directions  ;  but  when  a  strong  current  or  wind 
prevails,  they  are  carried  with  it  in  a  stream,  preserving  how¬ 
ever,  as  yet,  their  irregular  up  and  down  motion. 

10.  Its  formation  is  sometimes  very  rapid,  and  its  accumu¬ 
lation  very  thick  in  the  lower  strata  of  our  atmosphere,  but 
the  intensity  is  variable.  Whenever  rain  or  snow  falls,  this 
dust  is  precipitated  on  the  ground  by  it,  whence  arises  the 
purity  of  the  air  after  rain  and  snow  ;  but  a  small  share  is  still 
left,  or  soon  after  formed.  In  common  weather  it  deposits 
itself  on  the  ground  by  slow  degrees,  and  the  same  in  closed 

\  rooms.  It  forms  then  the  dust  of  our  floors,  the  mould  of  our 

roofs,  and  ultimately  the  surface  of  our  soil,  unless  driven  by 
winds  from  one  place  to  another. 

11.  I  have  measured  its  accumulation  in  a  quiet  room,  and 
have  found  it  variable  from  one-fourth  of  an  inch  to  one  inch  , 
in  the  course  of  one  year  ;  but  it  was  then  in  a  pulverulent 
fleecy  state,  and  might  be  reduced  by  compression  to  one- 
third  of  its  height,  making  the  average  of  j^early  deposit 
about  one-sixth  of  an  inch.  In  the  open  air  this  quantity  must 
be  still  more  variable,  owing  to ‘the  quantities  carried  by 
the  winds  and  waters  to  the  plains,  valleys,  rivers,  the  sea, 
&c.  or  accumulated  in  closed  places  or  against  walls,  houses, 
&c.  I  calculate,  however,  that  upon  an  average,  from  six  to 
twelve  inches  are  accumulated  over  the  ground  in  one  hun¬ 
dred  years,  where  it  mixes  with  the  soil  and  organic  exuviae, 

to  form  the  common  mould. 

12.  The  uses  of  this  chronic  meteor  are  many  and  obvious. 

It  serves  to  create  mould  over  rocks,  to  increase  their  de¬ 
composition,  to  add  to  our  cultivable  soil,  to  amalgamate  the 
alluvial  and  organic  deposits,  to  fertilize  sandy  and  unfruitful 
liracts  in  the  course  of  time,  to  administer  to  vegetable  life,  &c. 

31  ^ 


I 


400 


072877878 


Rajimsque  on  Atmospheric  Dust. 


It  does  not  appear  that  it  has  any  bad  influence  on  men  and 
animals  breathing  it  along  with  air,  unless  it  should  be  accu¬ 
mulated  in  a  very  intense  degree. 

13.  At  Segesta,  in  Sicily,  are  to  be  seen  the  ruins  of  an 
ancient  temple  ;  the  steps,  which  surround  it  on  all  sides  be¬ 
low  the  pillars,  are  built  on  a  rock,  on  the  top  of  a  hill 
detached  from  any  other  higher  ground.  Yet  now  all  the 
steps  and  the  base  of  the  pillars  are  under  the  ground,  which 
has  accumulated  from  this  dust  and  the  decay  of  plants  (not 
trees)  to  which  it  has  afibrded  food.  There  are  from  five  to 
eight  feet  from  the  rock  to  the  surface  of  this  new  soil,  which 
has  chemically  combined  in  a  variety  of  hardness.  This  soil 
has  arisen  there  in  about  2000  years,  notwithstanding  the 
washings  of  rain.  I  quote  this  as  a  remarkable  instance  of 
the  increase  of  soil  by  aerial  deposits,  among  many  which 
have  fallen  under  my  personal  examination. 

14.  It  is  commonly  believed  that  the  dust  of  our  rooms  is 
produced  by  the  fragments  of  decomposed  vestments,  bed¬ 
dings,  furnitures,  &c.  ;  this  cause  increases  it,  and  produces  a 
difierent  dust,  which  mixes  with  the  atmospheric  dust ;  but  it 
is  very  far  from  producing  it. 

15.  The  dust  of  the  open  air  is  ascribed  to  that  raised  from 
roads  and  fields,  by  the  pulverization  of  their  surface  ;  but 
this  secondary  and  visible  dust  is  only  a  consequence  of  the 
first.  From  whence  could  arise  the  dust  observed  by  the 
means  of  the  sunbeams  in  a  dark  corner,  in  winter,  when  the 
ground  is  frozen,  or  when  it  is  wet  and  muddy,  or  at  sea,  or 
on  the  top  of  rocky  mountains  ? 

16.  It  is  therefore  a  matter  of  fact,  worth  taking  into  con¬ 
sideration  by  geologists,  that  the  air  still  deposits  a  quantity 
of  dust,  which  must  have  been  much  greater  in  former  pe¬ 
riods.  Just  the  same  as  the  sea  deposits  still  a  quantity  of 
earthy  and  saline  particles  dissolved  in  it,  and  which  were 
superabundant  at  the  period  when  the  rocky  strata  were 
formed  on  its  bottom.  Water  being  more  compact,  deposits 
rocks.  Air,  which  is  less  dense,  deposits  a  pulverulent 
matter ! 


